Mobile systems and displays demand efficient and longer battery usage. Additionally, display quality is the most important performance feature that cannot be compromised even during heavy load current fluctuation, associated large drop-out voltage transition due to it and switching noise from the employed DC (direct current)-DC converters.
The active matrix OLED (AMOLED) display becomes very popular for mobile display applications owing to its advantages such as high display quality, low power consumption and low material cost. The AMOLED panel usually requires both positive and negative power suppliers with different regulated voltages, which are related to each display panel load. Each panel has different output current and voltage levels requirements based on the application such as panel size, pixel numbers, display quality, adopted process generation, and the like.
FIG. 1 shows a conventional single inductor AMOLED power supply, which is a two-stage SIBO converter. As shown in FIG. 1, the conventional two-stage SIBO converter 100 includes a synchronous boost circuit 120, a charge pump 140, an inductor L11 and capacitors C11-C15. The capacitors C11-C13 are decoupling capacitors. The capacitors C14-C15 are fly capacitors. The conventional two-stage SIBO converter 100 generates a positive output Vop for driving the load 160 by the current lop, and a negative output Von for driving the load 180 by the current Ion. The input provides the input voltage Vin and the input current Iin.
The synchronous boost circuit 120 is configured to boost the input Vin to the positive output Vop.
The charge pump 140 is configured to generate the negative output Von from the positive output Vop. The charge pump 140 has four steps, i.e. −1×, −0.66×, −0.5× and −0.33×. By using the fly capacitor C14, the charge pump 140 may implement the step −1×, that is, Von=Vop*(−1). By using the fly capacitors C14 and C15, the charge pump 140 may implement the steps −0.66×, −0.5× and −0.33×, that is, Von=Vop*(−0.66), or Von=Vop*(−0.5) Von=Vop*(−0.33).
From FIG. 1, the generation of the positive output Vop and the negative output Von are independently controlled.
FIG. 2 shows the conversion efficiency of the two-stage SIBO converter 100. The conversion efficiency is defined as:
  Efficiency  =                                                    Iop            *            Vop                                    +                                        Ion            *            Von                                                                  Iin          *          Vin                              *    100    ⁢    %  
As shown in FIG. 2, the efficiency of the conventional two-stage SIBO converter 100 is at peak when Von=Vop*(−0.5)=4.6*(−0.5)=−2.3(V) or Von=Vop*(−0.33)=4.6*(−0.33)=−1.51(V) in case that Vop=4.6(V). However, the efficiency of the conventional two-stage SIBO converter 100 is not good when Von is neither −2.3(V) nor −1.51(V). Therefore, the efficiency of the conventional two-stage SIBO converter needs to be improved.
There is a need for providing a new SIBO converter with improved efficiency.